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Abstract:
Here we presented a combination study of PM2.5 water-soluble ionic composition and PM2.5 acidity of aerosol samples collected from urban site (CNEMC) in Beijing, China, from 2016-2020, to assess the response of secondary inorganic species in PM2.5 to the emission reduction and reveal the characteristics and main driving factors of aerosol pH variation. The averaged SO42-, NO3-, and NH4+ concentration was 8.9, 16.6 and 8.2 μg m-3 at CNEMC site during autumn-winter seasons, driving the rapid increase of PM2.5 during pollution. The ISORROPIA II model was employed to predict PM2.5 pH in autumn-winter seasons, with an average of 3.9 (1.4~6.9) and 4.4 (2.4~6.9) during pollution and non-polluted periods, all acidic. Based on the continuous observations and simulation throughout 2020, PM2.5 pH showed a significant seasonal cycle pattern. Aerosols in summer were more acidic (pH range: 1-3), while aerosols in winter were less acidic (pH range: 3-6) and fluctuated more violently. We also conducted a sensitivity analysis of PM2.5 pH to changing meteorological parameters and SNA levels. Overall, temperature was found to be the main factor driving the seasonal and interannual variations in aerosol pH. This result may provide a scientific basis for formulating regional air quality improvement strategies, and provide a new perspective for exploring the influence of meteorological factors on atmospheric chemical processes.
